How to change angle between two faces of an object?
In Edit mode, on the bottom horizontal menu bar, select the Vertex as Snap Element mode (next to the Snap tool button), but no need to enable the Snap tool, select the vertices you want to move, press X to move them only along the X axis, keep pressing ctrl to temporarily enable the Snap option, drag your mouse to one of the vertices of the other edge, the vertices you've selected will move on the X axis and snap to the same position of the X axis than the other vertices.